If you just want to ride with your mates who aren't terribly fast... just get some 700c wheels for your MTB. You'll roll as fast as them, climb pretty much as well as they do. On fast flats and downhills you might get frustrated at running out of gears. If you do get frustrated by that - keep the wheels and build a road bike around them, if you don't then swapping back and forth between road and MTB wheels takes moments
The 700c wheels I bought for my inbred are now on a Merckx.